With all we know of Sonic 3 it's an interesting sprite timeline. I feel like it goes something like this: 1. There's a prototype that probably exists before this one that likely includes more of the beta Knuckles sprites and early versions of the S&K levels 2. Sonic Team decides that they want a new sprite style around when they realize Sonic 3 has to be split into 2 games. Knuckles is cut as a playable character. This beta is the inbetween where some aspects of their original design exist (the Angel Island Knuckles sprites, the surfboard intro, the hydrocity act 1/launch base act 1 backgrounds) and some new ideas exist (Knuckles new Sprite set) 3. Sonic 3 comes out as is. They weren't able to finish everything though, as is shown by Sonic's incomplete sprites and Tails' minimal changes. 4. The Sonic 3C builds are made. They're also when later stages started to be finalized as these stages have specific sprites for Tails that do not exist in Sonic 3 alone (the stage specific Sky Sanctuary Tails sprites don't exist in Sonic 3 alone, nor do his super transformation sprite). 5. Sonic & Knuckles comes out. Some Tails sprites exist in S&K due to how the game ended up.